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CHAPTER 2:: ATOMS, MOLECULES AND IONS Atom: Basic unit of matter. Smallest particle of the element which retain the properties of that element. Eg: Oxygen, Hydrogen Molecules: Two or more atoms (same or different type of atoms) join together by chemical force to form molecules. Avogadro’s Hypothesis At the same temperature and pressure, equal volumes volum of different gases contain the same number of particles. Example: 5 liters of oxygen and 5 liters of nitrogen contain same number of particles! - But it is not true (because different atoms have different size and properties). To understand about the atoms, we need to know about what atoms made of, and how do the various atoms differ? Eg: H2, H2O Ions: Atoms or group of atoms (molecules) with positive or negative charge. + - + Eg: Na , Cl , NH4 Fundamental Chemical Laws Law of Conversion of mass: In a chemical reaction, mass (weight) is neither created nor destroyed. Early experiments to characterize the atom 1) Thomson model He postulated that “an atom consisted of a diffuse cloud of positive charge with the negative electrons embedded randomly in it”. This model is called as palm-pudding model. Mass of reactant is equal to mass of product. Law of Definite Proportion: A given compound always contains exactly the same proportion of elements by mass. Law of Multiple Proportions: When chemical elements combine to form a compound, they do so in a ratio of small whole numbers. 2) Millikan experiment Millikan found out that the mass of electron is 9.11 X 10 -31 kg. 3) Rutherford experiment Dalton’s atomic theory 1) Each element is made up of atoms. 2) Element are formed when same type of atoms join tog together. He explained that atom has • A center of positive charge (nucleus) • Electrons moving around the nucleus. • It is called as nuclear model. 3) Compounds are formed when atoms of different elements combine. 4) Chemical reactions Involve reorganization of atoms. During reaction, only the way atoms bound together changes changes, but there is no change in atom.. General Chemistry – Handout_Ch-2 (Page: 1) ATOMIC STRUCTURE Element Symbol: One or two letter abbreviation. It is based on current name or original (old) name. Current Name Original Name Symbol Hydrogen - H Carbon - C Calcium - Ca Oxygen - O Iron Ferrum Fe Copper Cuprum Cu 1) Protons present in the nucleus. It has positive chare. Silver Argentum Ag 2) Neutrons present in the nucleus. It has no charge.. It also has almost the same mass as a proton. Sodium Natrium Na Atom consist of 3) Electrons move around the nucleus. It has negative charge. Protons determine the identity of an atom (atoms identified depending on the number of protons present in it). Protons and neutrons are present in the tiny nucleus at high density. Both of them accounts for almost all the atom’s mass (atomic mass) mass). Atomic number (Z) = Number of protons (or electrons) Mass number (A) = Number of protons + Number of neutrons The atomic number (Z, number of protons) is written as a subscript to the left of the abbreviation,, while the Mass number is written as a superscript on the left. Electrons occupy most of atom’s volume. Mass number (A) Electrons form the outer shell of the atom, so that it allows one atom to interact with another. Thus, it is responsible for the chemical properties of the atom. Atomic number (Z) Atoms are neutral in charge, because the number of protons is equal to number of electrons. Example: Element Symbol Carbon Abbreviation: C Atomic number is in subscript: 6C ATOMIC SYMBOLS Mass number is in superscript: To identify the different elements (atoms), we have a standard way of representing each element. The element (carbon) is written as: 12 C 12 6C Element symbol with atomic number and mass number number. General Chemistry – Handout_Ch-2 (Page: 2) Surprisingly, in periodic table only atomic number (Z) is mentioned along with the abbreviation of the element (no mass number). Example: Carbon is written as 6 C Uses of atomic symbol You can find out atomic number and mass number from atomic symbol symbol. Example. 23 11Na Atomic number (Z) = 11 Mass number (A) = 23 From the atomic symbol, you can also calculate the number of electron, proton and neutron. Example. 23 11Na Number of proton Example: 2 He = 11 (because Z = number of proton) Atomic number (Z) = 2 Number of electron = 11 (because it is equal to number of protons) Number of protons = 2 Number of neutrons = Mass number – atomic number Number of electrons = 2 = 23 - 11 = 12 Uses of periodic table If the element name (abbreviation) and mass number is given, you can write the atomic symbol. All the elements are arranged in periodic table depending on the atomic number (Z). Example: ‘Na’ has mass number 23. Write W the atomic symbol. Atomic number of Na = 11 (find out from the periodic table) From the periodic table you can find out number of protons and number of electrons present in an atom. Mass number = 23 Atomic symbol = 23 11Na General Chemistry – Handout_Ch-2 (Page: 3) ISOTOPES 34 Atoms with the same atomic number but different mass numbers are called isotopes. Isotopes have same number of proton and electron, but different number of neutron. Because the chemical property of the atom depends on electrons, the isotopes have same chemical property. 23 11Na. If sodium has 13 neutrons it is written as 24 11Na. 23 11Na and 24 11Na - SIMPLE INTRODUCTION TO PERIODIC TABLE 1. From left to right (row of periodic table) the atomic number (Number of protons) increases. 2. From top to bottom (vertical column of periodic table), the elements have similar chemical properties. are called as isotopes. IONS Atoms or group of atoms (molecules) with a net positive or negative charge is called ions. + 34 Chlorine ( 17Cl ) has 17 protons (positive charge) and 18 electrons (negative charge). So it has net negative charge (+17-18 (+1 = -1). As the different elements were discovered, they are arranged in a table depending on the atomic number and chemical properties. This is called periodic table. Example: Sodium (11Na) has 11 electrons and 11 protons. If sodium has 12 neutrons it is written as Chlorine ( 17Cl) has 17 protons (+ve charge) and 17 1 electrons (-ve charge). So it is neutral (+17-17 = 0). It is very ery important to know the metals and non-metals non in periodic table (very helpful in naming simple compounds. - Example. Na (net positive charge), Cl (net negative charge) Cations: Positive ions are called cations.. They are produced by the removal of one or more electrons. 23 11Na Example. ---------- Removal of one electron ---- 23 + 11Na 23 Sodium ( 11Na) has 11 protons (+ve charge) and 11 electrons ((-ve charge). So it is neutral (no charge). 23 + Sodium ion ( 11Na ) has 11 protons (+ve charge) and 10 electrons ((-ve charge). So it has net positive charge (+11-10 = +1). Anions: Negative ions are called anions. They are produced by the addition of one or more electrons. Example. 34 17Cl ---------- Addition of one electron ---- 34 17Cl General Chemistry – Handout_Ch-2 (Page: 4) Classification of Ions (depending on number of atoms): atoms) Metals • Most of the elements are classed as metals because they conduct heat and electricity. • Metals frequently lose electrons to form positive ion ions. • It can form ionic compound with a negatively charged non non-metal. • Metals are in the left side and middle of the periodic table. Non-metals • Non-metals metals frequently gain electrons to become negatively charged. • They can form ionic compound with a metal ion. • They also can form covalent compound by bonds with each other. • Non-metals are in the upper right side of the periodic table (Diagonal from B to At) and H. Ions are classified into monoatomic ions and polyatomic ions depend on the number of atoms present. Monoatomic ion: An atom which has net positive or negative charge. + + Example: H , Na , Cl Polyatomic ion: Group of atoms (more than 1 atom) which have net positive or negative charge + Example: NH4 , NO3 atoms) Classification of Ions (depending on number of atoms): Ions also classified based on the capacity of the metal met to form one or more cations, Type I: Metals forms only one cation. These metals present in the + 2+ 2+ 3+ 3+ left side of the periodic table (and Ag , Zn , Cd , Al , Ga ). + 2+ 3+ Example. Na form only Na (not Na or Na ). COMPOUNDS Two or more different chemical elements join together by chemical force (chemical bond) to form compounds. Eg. + Na + Cl - + NaCl - NH4 + NO3 Type II: Metal forms more than one cation. These metals present in the middle of the periodic table. 2+ 3+ Example. Fe can form Fe and Fe NH4NO3 Classification of compounds 1. Covalent compounds are formed ormed by sharing electrons between non-metals. Ga Example: 3+ H + H → H2 2. Ionic compounds Consist of positive (metal cation) and negative (non-metal anion) charged ions were attracted together because of opposite charge. Compounds made in this manner are also called as ionic solids or salts. Example: + - Na + Cl → NaCl General Chemistry – Handout_Ch-2 (Page: 5) MOLE (also written as mol) Mass number (A) 1 mole of a substance = 6.022 x 10 Mass number = Number of Protons + Number of Neutrons 23 atoms (or) units 23 6.022 x 10 is Avogadro's number. It is indicated in the top left side of the element symbol. Example: Example 12 1 mole of carbon = 6.022 x 10 6C: Mass number of C (carbon) = 12 23 carbon atoms 1 mole of hydrogen = 6.022 x 10 1 mole of CH4 = 6.022 x 10 Atomic mass (atomic weight) Atomic mass is the mass of an atom in atomic mass units (amu). Atomic mass = Mass number in amu (unit) 23 23 hydrogen atoms CH4 molecules Solution 1: How many moles of oxygen atoms are there in 10 moles of KClO3? 1 amu = Mass of 1 proton or 1 neutron 1 KClO3 molecule contains 3 oxygen atoms. Example: 1 mol of KClO3 contains 3 mol O Mass of 12 C = 12 amu Therefore, 6 10 mol of KClO3 contains 30 mol O Average atomic mass Elements present in nature as mixtures of isotopes (same numbers of protons but different numbers of neutrons). Example: Measuring moles 1 mole of element = Mass number in grams Carbon Example: 98.89% present as 1.11% present as 12 C 1 mole of 12 C (carbon) = 12 grams of 12 C 13 C < 0.01 % present as 14 C Atomic mass of C = (12 x 98.89) + (13 x 1.11) + (14 x 0.01) 100 = (1186.68 + 14.43 + 0.14)/100 = 12.0125 = 12.01 amu Thus, 12 gram of 12 C (carbon) = 6.022 x 10 1 23 1 gram of H (hydrogen) = 6.022 x 10 23 carbon atoms hydrogen atoms We can find out the relation between atomic mass unit (amu) and gram (g). 6.022 x 10 23 carbon atoms = 12 grams 1 carbon atom = 12 amu Atomic mass can be determined using an instrument called ‘Mass Spectrometer’. (6.022 x 10 23 atoms) x (12 amu / 1 atom) = 12 g = 6.022 x 10 23 amu = 1 g General Chemistry – Handout_Ch-2 (Page: 6) MOLAR MASS Solution 3: How many H atoms are in 72.5 g of C3H8O? Molar mass = Sum of the masses of 1 mole of constituent atoms (in grams). 1 mol C3H8O = (3 x 12) + (8 x 1) + 16 = 60 g C3H8O Example: Convert g to mol (Mol = Mass / Molar mass) CO2 contain one C and two O atoms (or) Moles of C3H8O = 72.5 g / 60 g 1 mol of CO2 contains 1 mol C and 2 mol O. = 1.21 mol Mass of 1 mol C = 12.01 grams 1 mol C3H8O molecules contain 8 mol H atoms Mass of 1 mol O = 16.00 grams 1.21 mol C3H8O molecules contain = 1.21 x 8 mol H atoms = 9.68 mol H atom Molar mass of CO2 = (1 x 12.01) + (2 x 16.00) grams 1 mol = 6.022 x 10 = 44.01 grams 23 atoms/units 9.68 mol H = 9.68 x 6.022 x 10 = 5.83 x l10 Solution 1: Calculate the molar mass of CaCO3 1 mol of CaCO3 contains 1 mol Ca, 1 mol C and 3 mole O. 24 23 H atoms H atoms Solution 4: How many grams do 8.50 x 10 1 mol Ca = 40.08 g 6.022 x 10 1 mol C = 12.01 g 23 22 molecules of NH3 represent? NH3 atoms/units = 1 mol Convert number of molecules to moles 3 mol O = 3 x 16 = 48 g 1 mol of NH3 = 14.01 + (3 x 1.008) = 17.034 g 1 mol of CaCO3 = 40.08 + 12.01 + 48 = 100.09 g Solution 2: Calculate the molar mass of (NH4)2Cr2O7 1 mol NH3 = 6.022 x 10 8.50x10 22 NH3 molecules x 1 mol of (NH4)2Cr2O7 contains 2 mol N, 8 mol H, 2 mol Cr, and 7 mole O. 23 NH3 molecules 1 mol NH3 x 17.037 grams 23 1 mol NH3 6.022x10 molecules of NH3 = 2.40475 grams 2 mol N = 2 x 14.01 = 28.02 g 8 mol H = 8 x 1.008 = 8.064 g 8.50 x 10 22 molecules of NH3 = 2.40 grams 2 mol Cr = 2 x 52 = 104 g 7 mol O = 7 x 16 = 112 g 1 mol of (NH4)2Cr2O7 = 28.02 + 8.064+ 104 + 112 = 252.084 g General Chemistry – Handout_Ch-2 (Page: 7) Sample Questions: 6) Which one of the following is highly responsible for the chemical properties of an atom? 1) Which of the following is true? a) An individual atom is a solid a) Electron b) Proton b) An individual atom is a liquid c) Neutron d) None of the above c) An individual atom is a gas d) An individual atom cannot be considered to be a solid, liquid or gas 2) The weight of a container with some chemicals is 250 g. If the chemicals are burned in a closed container, which one of the following is true? a) Weight decrease less than 250 g change (250 g) b) Weight do not c) Weight increase more than 250 g depends on the chemical d) Weight change 7) How is an ion formed from a neutral atom? a) Adding or subtracting protons subtracting neutrons c) Adding or subtracting electrons a) Atomic mass b) Number of neutrons c) Alphabetical order d) Atomic number 9) How many protons, neutrons and electrons are in Atomic mass = 235 Number of protons = 92 Number of electrons = 92 46 g + 96 g = 54 g + Mass of carbon dioxide Number of neutrons = 235 – 92 = 143 Calculate the number of protons, electrons and neutrons for the following atoms. 4) Which one of the following has no charge? b) Proton c) Neutron d) All the above 5) Which one of the following determines the identity of an atom? a) Electron b) Proton c) Neutron d) All the above 23592U? Atomic number = 92 Mass of Ethanol + Oxygen = Mass of Water + Carbon dioxide a) Electron d) All the above 8) Elements were arranged in periodic table depending on ……… 3) In a combustion reaction, 46 g of ethanol reacts with 96 g of oxygen to produce water and carbon dioxide. If 54 g of water is produced, what mass of carbon dioxide is produced? Mass of carbon dioxide = (46+96) – 54g = 88 g b) Adding or Atom 208 Pb 82 No. of Protons No. of Electrons No. of Neutrons 82 82 208-82 = 126 13 6C 6 6 13-6 = 7 19 9F 9 9 19-9 = 10 41 20Ca 20 20 41-20 = 21 235 U 92 92 92 235-92 = 143 General Chemistry – Handout_Ch-2 (Page: 8) 10) How many protons and electrons in ‘He’? 13) What is the atomic symbol (AZX) for Z=8, and number of neutron = 9. Atomic number of He (see the periodic table) = 2 If Z = 8, Element name = O (see the periodic table) Number of protons = 2 Number of protons = 8 (from Z, atomic number) Number of electrons = 2 Number of neutrons = 9 Calculate the number of protons and electrons for the following atoms. Atom No. of Protons Atomic number (from the periodic table) Mass number (A) = 8 + 9 = 17 Atomic symbol = No. of Electrons 17 O 8 14) How many numbers of protons and electrons present in Ba2+ ion. Mg 12 12 12 Zn 30 30 30 11) How many protons, neutrons and electrons are in Atomic number of Ba (see the periodic table) (Z) = 56 Number of protons in Ba2+= 56 Number of electrons in Ba2+ = 56-2 = 54 79Br? For each of the following ions, indicate the number of protons and electrons. Atomic mass (A) = 79 Atomic number (see the periodic table) (Z) = 35 Atom Z No. of Protons 30 30 30-2 = 28 7 7 7+3 = 10 35 35 35+1 = 36 Number of protons = 35 Number of electrons = 35 Zn 2+ Number of neutrons = 79 – 35 = 44 N3- Calculate the number of protons, electrons and neutrons for the following atoms. Atom A Z No. of Protons No. of Electrons 133 55 55 55 133-55 = 78 81Br 81 35 35 35 81-35 = 46 56Fe 56 26 26 26 56-26 = 30 133Cs Br - No. of Electrons No. of Neutrons 12) What is the atomic symbol (AZX) for Z=27 and A = 60. If Z = 27, Element name = Co (see the periodic table) Atomic symbol = 60 Co 27 General Chemistry – Handout_Ch-2 (Page: 9)